Homodyne signal detection systems and methods detect frequency-modulated radiation by mixing a signal with a reference signal. For example, a laser beam may be split into two portions. One portion is transmitted to a target, while another portion is a reference signal that is compared to a return signal that reflects from the target.
A typical homodyne detection system determines a location of a target by detecting a phase difference between two light beams having the same wavelength. In contrast, A typical heterodyne detection systems determines a location of a target by detecting a frequency difference between two light beams.
Light detection and ranging (LIDAR or LADAR) systems may be used to detect surface features of a target, such as various areas on the surface of the Earth. A typical LIDAR system includes a laser, a scanner, and a detector. The laser emits light pulses that are used to measure distances with respect to various areas of a particular target. The scanner moves the light pulses over the surface of the target. The light pulses reflect off the target and are received by the detector. The reflected light pulses received at the detector may be used to generate three-dimensional information about the surface shape and area of the target. Similarly, radio detection and ranging (RADAR) systems use radio waves to determine a range, angle and/or velocity of a target. LIDAR and RADAR may be homodyne or heterodyne based systems.
Resolution of known active measurement systems, such as LIDAR systems, is typically limited, due to the noise at a fundamental frequency level. For example, a known LIDAR system may use homodyne detection in which a coherent signal is transmitted to a target. The signal is scattered from the target and returns to a detector. Prior to arriving at the detector, the weakened return signal is combined with a stronger coherent signal. However, quantum noise (for example, vacuum noise) inherent within such a system decreases the clarity and resolution of the detected signal.